Several thousand people protested in Skopje against an agreement that would ensure the wider use of the Albanian language in the ethnically divided state.
Last Thursday, the leader of the Social Democrats, Zoran Zaev, said he expected to be able to form a government in March after he had secured support from ethnic Albanian parties in the 120-seat parliament.
Those parties had made their support for any potential coalition conditional on the passage of a law backing broader use of their language in Macedonia.
But on Monday, a movement that called itself "For Joint Macedonia" called on social media for people to come out on the street and protest the deal Zaev had made with the Albanian parties.
Protesters marched from the government building to the state parliament in Skopje shouting "This will not pass" and sang Macedonian national songs.
"With one symbolic gesture we want to show how you should love Macedonia," said Bogdan Ilievski, a member of the movement.
The Balkan nation's two-year-old political crisis was triggered by a surveillance scandal that forced veteran leader of the nationalist VMRO-DPMNE, Nikola Gruevski, to resign a year ago.
The crisis was the worst since Western diplomacy helped drag the country of 2.1 million people back from the brink of civil war during an ethnic Albanian insurgency in 2001, promising it a path to membership of the European Union and of NATO.
In a snap vote in December, VMRO-DPMNE won 51 seats to the Social Democrats' 49, and neither was able to form the government without parties representing ethnic Albanians who make up one third of the population.
The conservative VMRO-DPMNE party had tried but failed to form a coalition.
On Monday Zaev asked President Gjorge Ivanov to give him the mandate to form a government and had presented him with the signatures of 18 deputies from ethnic Albanian parties.
On Sunday evening former prime minister Gruevski called on Social Democrats to revoke the deal, saying it was unconstitutional and jeopardised state interests.
Albanian is currently an official language only in municipalities where Albanians account for more than 20 percent of the population.